1. Field of the Invention
The present invention relates to a composite material blade for an aircraft rotor with rotating wings and the invention relates more precisely to a blade for equipping helicopter rotors in particular for military use and whose twin longeron structure extending over the whole span of the blade and whose twin torsion and flexion box structure extending over the whole chord of the blade, as well as laminated honeycomb sandwich coverings, provide a fail-safe characteristic.
Numerous constructions of composite material blades are already known for helicopter rotors, whose structure has been particularly designed for withstanding torsional and flexional forces and for being redundant so as to preserve the integrity of operation after structural damage.
In general, the structure of these known blades includes a main leading edge longeron made from laminated threads of high mechanical strength fibers agglomerated by means of a synthetic polymerized resin, possibly doubled by one or more other auxiliary longerons, of the same construction, and either a single box in the vicinity of the leading edge, integrating the main longeron and one or more of the auxiliary longerons if provided, or a multiple box extending along the chord or in the thickness of the blade and integrating all the longerons. The or each box of these blades contains a working filling core formed of a block of a honeycomb of cellular material of average density, such as a foam, or a honeycomb block and is enveloped at least partially by the skin of the blade generally formed from a stack of at least two layers of a fabric of fibers agglomerated by a synthetic polymerized resin. The fibers are possibly oriented in different directions, preferably crossed, and slanted with respect to the longitudinal axis of the corresponding blade. In the case of a structure with a single box at the leading edge, the rear part of the blade includes one or more filling blocks, adjacent to the forward box and situated therebehind and possibly adjacent each other depending on the span of the blade, and this or these filling blocks, enveloped directly by the skin of the blade, is or are formed either from an alveolar or cellular material, for example with closed cells such as synthetic foam, or as a honeycomb. In all cases, the main advantage sought is a great rigidity under torsion, more particularly for eliminating the vibrations resulting from coupling between the flexional and torsional deformations of the blade.
2. Description of the prior Art
By way of example, the French patent No. 2 381 662 of the applicant relates to a composite material blade with forward torsion box formed of a leading longeron extending over the whole span of the blade and formed as a single piece, from hanks of laminated threads of glass fibers, with a solid cross section substantially in the form of a slightly flattened D, having a solid core made from a light alveolar material and juxtaposed with the whole rear face of the longeron and a layer surrounding the longeron and the core made from at least one layer of a fabric including parallel glass fibers disposed slantwise with respect to the longitudinal axis of the blade, this layer having considerable thickness, appreciably greater than that of the coating or skin of the blade, which envelops the forward box as well as a rear filling element, also made from a light alveolar material and juxtaposed against the cross and central rib formed by the rear face of the box, that is to say by the part of the peripheral layer which covers the rear face of the core of the box, this latter extending along the chord of the cross section of the blade over approximately the front half of the profile of the blade, whose external shape it matches.
Other similar constructions are known, in which the rear filling element is a honeycomb block.
By way of second example, the French patent No. 84 19 432 of the applicant describes a multilongeron and twin box structure whose resistance framework, close to the leading edge of the blade, is formed as two boxes superimposed in the direction of the thickness of the blade, each of which extends over the whole span of the blade and is formed of a front longeron and a rear longeron each formed of a bundle of laminated threads of fibers agglomerated by a resin, a filling core made from a light material disposed between these two longerons and an envelope enclosing the two longerons and the core, and including at least one layer of a fabric made from parallel fibers disposed slantwise with respect to the longitudinal axis of the blade, the two boxes being firmly secured together by simultaneous polymerization of the superimposed and jointing parts of their envelopes, thus forming a median sole, whereas a light alveolar filling element is disposed at the rear of the two boxes and an outer coating envelops the two boxes and the rear filling element.
The disadvantages of these known constructions in which the cores of the box or boxes and or the rear filling elements are honeycomb blocks, concern their complexity of manufacture and their cost, for their manufacture requires great machining accuracy and precautions and tight tolerances so that the honeycomb blocks have upper and lower surfaces which are at all points "parallel" to the profile of the desired blade, while ensuring suitable clamping of the honeycomb blocks by their covering, for if they are clamped too much the hexagonal mesh or cells of the honeycomb network is crushed whereas insufficient clamping results in separation between the coating and the filling and/or surface defects. The perfect machining required for the honeycomb blocks is all the more complicated since modern blades have profiles which are evolutive depending on the span, even tapering. Another drawback of these known constructions is that the honeycomb filling cores or elements do not provide a sufficient internal counterpressure during molding of the blade for their stiffness is practically zero in the direction of the chord of the blade. It is therefore impossible to manufacture, in a single molding operation, a blade having an economical longeron of the type formed of hanks of laminated threads of high mechanical strength fibers impregnated with polymerizable resin, and which is shaped during polymerization of the resin, during molding, through the presence of a filling and/or a core exerting a counter pressure ith respect to the molding pressure caused by closure of the mold parts. Similarly, it is impossible to manufacture in a single molding operation a twin box blade having therefore a central rib, or even worse, a multibox blade with several ribs for this or these ribs must also be shaped with the assistance of a sufficient counterpressure provided by a filling core and/or element.
These drawbacks are not to be found in known constructions in which the cores of the box or boxes and/or the rear filling element or elements are made from a light alveolar or cellular material, such as a foam, for such a material plays the role of internal shaping tool or mandrel, providing an internal counterpressure during molding, which gives the possibility of manufacturing blades whose internal structure may be complex, in a single molding operation. The interest of this advantage is essential for designing an economic blade. In addition, because of its flexibility, such an alveolar material, similar to a foam, requires much less accurate machining than the honeycombs, therefore less costly and a sufficient overthickness of the prefabricated foam blocks allows any surface defects to be avoided.
On the other hand, these known constructions with box core and/or rear filling elements are made from an alveolar or cellular material such as a foam have drawbacks which are proper thereto, particularly in the case of shocks or impacts on the blades. These shocks or impacts cause deformation of the coating and of the filling of the blade, and these deformations are the cause of stripping phenomena about the periphery of the impacts, which results in considerable separations which only stop, in the case of violent shocks, after reaching a resistant element such as the trailing edge of the blade or an edge or a wing of a longeron, and this whatever the method of connecting the coating to the filling or foam core (adhesive film or self adhesive resin interface or not), for the rupture always occurs in the foam. In fact, it is the very nature of these alveolar materials which does not allow them to withstand and transmit stripping loads, contrary to honeycombs which support this type of stress well, because of the good connection obtained at the level of each mesh or cell of the hexagonal network with the coatings, by means of adhesive films.
Furthermore, the known constructions of blades with thin laminated coatings, formed by a simple stack of a few layers of fiber fabric and in which the box cores and/or rear filling elements are made from alveolar materials or from honeycomb blocks have as common disadvantages that these blades are necessarily dependent, particularly with respect to shocks and impacts, on the stabilization provided by the cores and/or the filling elements, and that in the case of damage following a shock or an impact of a projectile, the repair requires the localized replacement of the material of the core of the filling over the whole thickness of the profile of the blade. In fact, whether this material is alveolar or honeycombed, in the case of perforation of a simple coating or thin skin of the blade, at the present time it is necessary to remove the whole of the damaged zone and, so as to be certain to find a healthy filling, the damaged filling must be replaced as far as the other face of the coating so as to avoid having to join the added piece with the filling in place, in the middle of the profile of the blade. In addition, in the case of a shock or impact during flight, the coating or thin skin becomes flexible and floating after unsticking from the filling, in particularly made from foam, and may rapidly, through the combined effect of aerodynamic stresses, vibrations, pressures and depressions, split and strip, thus presenting a real risk.
As state of the art the French patent No. 1 154 586 may also be mentioned which describes a blade whose covering on the rear part is formed of two intercalated panels with internal and external metal walls, between which is disposed a light filling made from honeycomb or other light filling material. But this blade has a front part and a rear part which are practically hollow, the front part including essentially a metal laminated leading edge longeron, of C cross section, formed of a large number of layers of thin metal sheets alternating with layers of an adhesive material, and a vertical longeron plate closing the rear part of the longeron, whereas the rear part of the blade includes stiffening elements in the form of the above mentioned intercalated upper and lower panels. The connection between the longeron and the longeron plate and between this latter and the walls of the intercalated panels are provided by laminated brackets and the intercalated panels, profiled with special radii of curvature and tapering at the front as at the rear, possibly include dense layer inserts, laminated and in stepped formation, between the walls of the panels which are such that the internal wall of one of the panels extends towards the trailing edge until it meets the external walls of the other panel, or such that the external walls of the panels extend as far as the trailing edge whereas their internal walls meet up forward of the trailing edge so as to form a joint, with, in any case, at least one profiled relatively rigid wedge shaped piece, providing, at the rear, the junction of the two covering panels. The drawbacks of this construction are related to the metal nature of the resistant framework and of the skin of the blade, as well as to the complexity of its structure, and so the complexity and to the cost of its manufacture, because more particularly of the need to machine the filling elements of the panels when they are made from honeycombs. In addition, the wedge shaped piece of the trailing edge indispensable to the strength of the blade and required in this construction for increasing the drag rigidity, and which is also a working member taking up a part of the centrifugal force, is a factor of vulnerability of the blade, for a single direct impact on this piece may cause its breakage.